function [P]=piston_3(WAD,kk,P_l,Ma_l,a_l,WAD_rigid)                          % 算
P=(1+kk/a_l*(Ma_l*a_l)*(WAD-WAD_rigid)+...
    kk*(kk+1)/(4*a_l^2)*(Ma_l*a_l)^2*(WAD-WAD_rigid)^2+...
    kk*(kk+1)/(12*a_l^3)*(Ma_l*a_l)^3*(WAD-WAD_rigid)^3)*P_l;
end